In general, projectors provide images by generating the image in a light source and projecting the same onto a screen. Referring to FIG. 1, a typical rear projection system or projection screen device includes a cabinet 110, a screen 140 installed on the front surface of the cabinet and where an image is formed, a light source 120, installed in the cabinet and generating and projecting an image, and reflection mirrors 100 and 130 reflecting the image input from the light source toward the screen. In the rear projection system having the above structure, an image projected in the rear of the screen and formed on the screen is viewed in front of the screen, that is, outside the cabinet.
Traditional rear projection televisions, as depicted in FIG. 1, generally are bulky, heavy, complicated to use, and expensive. A rear projection television or projection screen device having a flat panel display using projection technology, as shown in FIG. 2, is ultra-thin, lightweight, and has the potential to save costs by using fewer components.
Rear projection screens are made of either a lenticular lens or a diffuser or a combination of a lenticular lens and a diffuser that distributes or spreads the incident light in some angular distribution. Typically, a polymer diffuser may comprise a matrix and a plurality of dispersed light-scattering centers, with a controlled refractive index differential between the matrix and the light-scattering centers. FIG. 3 depicts a rear projection screen without a fresnel lens. Before the screen 310, incident light 320 has a certain incident angle. Beyond the screen, the strongest component of the distributed light 330 is in the incident direction. As such, a viewer sees a varying brightness on the screen, resulting in an uneven image where the light is brightest in the center of the screen and darkest in the corners of the screen. In projection with a refractive-type fresnel lens, depicted in FIG. 4, the fresnel lens 420 redirects the incoming light 410 such that its incident angle, as well as the strongest component of the distributed light 430, are both normal to the projection screen 440. This gives a more even brightness to the screen. Accordingly, even when a viewer moves to the edge of the screen, the level of brightness at different positions decreases more evenly.
FIGS. 5 and 6 illustrate a projection television employing a fresnel lens. Such projection televisions have great depth. FIGS. 7 and 8 show a newer traditional projection television with an off-axis fresnel lens. The depth of such a projection television is considerably thinner due to the off-axis fresnel lens, which permits the light source to be directed from below.
The general fresnel lens structure can be conceptualized as either a collection of grooves between facets, or a collection of facets between grooves. With reference to the cross-sectional view of a fresnel lens shown in FIG. 22, the fresnel lens facets can be described with two angles. The face angle 2220 (also called “facet angle”) is defined as the angle between the surfaces adjacent grooves. The groove angle 2210 is the angle formed between the input face (i.e. the bottom side) of one facet and the reflection face (i.e., the top side) of the same facet. The geometry of a facet having a curved side, as in the present invention, is described below.
FIG. 21 shows a conceptual illustration of the sections of a larger fresnel lens. The sections can be used for rear projection screens. The fresnel lens has an axis 2130 at the center of a plurality of concentric rows of facets and grooves having predetermined facet and groove angles. In a rear projection display device in which incoming light enters perpendicularly to the face of the lens, or where the full lens field of the projection lens system is used, a center portion (indicated by the dashed rectangle) 2120 of fresnel lens 2100 is used as a fresnel lens for the display device. Rectangle 2110 provides an indication of a screen displaced from the center portion of fresnel lens 2100, as used in off-axis fresnel lenses where the incoming light enters at an angle. The size and shape of the portion of the lens to be used corresponds to the size and shape of the screen of the display device, i.e., the projection display. The term “off-axis” is used because the physical center of the fresnel lens 2110 is displaced from the axis 2130 of the larger fresnel lens 2100. In an off-axis lens, only the displaced portion is used. Any remaining portion of a larger fresnel lens from which the off-axis lens may have been derived is not used in the off-axis lens. Alternately, manufacturing techniques exist whereby only the off-axis portion of the fresnel lens is manufactured.
Although only the displaced portion of the lens is used in an off-axis fresnel lens, the off-axis lens is still considered to have an axis. The axis, however, may not appear on the actual lens. However, its position may be extrapolated from the elongated and arcuate concentric facets and grooves of the fresnel lens structure. For example, the off-axis fresnel lens 2110 has an axis 2130, even though the axis 2130 is at the lower edge of the lens. In other embodiments, the axis of an off-axis fresnel lens may even be substantially below or otherwise outside the lens border.
Though it may not be visible on the lens itself, the axis of an off-axis fresnel lens can be extrapolated by determining the radius of a circle defined by any one of the concentric rows of facets.
FIG. 20 shows a side view of a rear projection television with an off-axis fresnel lens. In FIG. 20(a), the light source 2000 is positioned below the screen 2010 having a height H and the incoming light rays strike the input surface of the screen at angles from the lowest ray angle D to the highest ray angle F, with a middle ray angle E. However, an off-axis lens may be used with any projection system where the light source is displaced from the center of the screen. FIG. 20(b) shows a front view of the projection screen with height H and width W. Specific dimensions of screen geometry and light incident angles for one embodiment are shown in FIG. 20(c).
FIG. 9 depicts the limited bending ability of a refractive-type prism, whereby the angle of bending δ is approximately half the prism angle θ. Because the bending angle is limited to only half of the prism angle, the projection angle is limited, which limits the thinness of the projection system. Moreover, when θ is large, reflection loss 910 becomes large. The light angle as well as loss depends on wavelength, thereby resulting in a color shift on the display screen.
FIG. 10 illustrates the greater bending ability of a reflective-type prism, whereby light is more fully reflected at the interface of the prism and air because of “total internal reflection” (“TIR”). While this kind of internal reflection is termed “total,” it should not be construed as absolute, as slight reflective loss may occur due to abnormalities or impurities in the prism material, interference of the light with air or other substances, or for other reasons which may be apparent to one skilled in the art. Nonetheless, TIR has a reflection efficiency nearing 100%. The bending angle δ could reach 90°, thereby making the projection system even thinner. A higher output brightness results because of less reflection loss. In addition, there is virtually no color shift because the bending angle and loss have no wavelength dependence.
FIG. 11 shows that a reflective fresnel lens has low resolution and scrambled images. That is, image resolution is limited by the distance Σ between facets, and the image on every pitch is scrambled. Accordingly, the sequence 1-2-3-4 in the input light rays 1100 becomes 2-1-4-3 in the output light rays 1110.
Another problem associated with fresnel lenses and projection screens is the reduced contrast due to ambient light. On the projection screen, dark colors are represented by an absence of light. Thus, any ambient light on the projection surface will decrease contrast by causing dark colors to appear lighter. This ambient light can originate from the input surface or output surface of the fresnel lens. In addition, depending on the configuration of the fresnel lens facets, light that is reflected within each TIR facet might not be optimally focused prior to reaching the projection screen.
As for the projection screen, most existing screens utilize some variation of a lenticular lens, wherein the exit surface of the projection screen is not flat (e.g., it is sinusoidal). Such non-flat viewing screens are quite prone to being damaged and difficult to handle, clean, and maintain. In addition, in existing systems, the input surface of the projection screen and/or the output surface of the fresnel lens are not flat. Therefore, when placed adjacent one another, or connected to one another, the fresnel lens-projection screen assembly inherently includes two additional surfaces (i.e., the output surface of the fresnel lens and the input surface of the screen) for reflecting ambient light, thereby causing loss of contrast on the viewing side of the projection screen.
Thus, there is a need for a reflective fresnel lens system that has high resolution, corrects the problem of image scrambling, has improved contrast, and optimizes the ability to focus reflected light rays within relatively small tolerances. Moreover, there is a need for a projection screen that can be easily and efficiently assembled with such a fresnel lens while, at the same time, optimizing contrast and maintainability.